1. Field of the Invention
This invention relates in general to a multistage detector in wireless communication systems, and more particularly to a method and apparatus for providing differencing multistage detection in the reverse link of a Code Division Multiple Access communication system.
2. Description of Related Art
Cellular systems have had a direct effect on the lives of millions over the past few years. For the first time, people are able to make and receive phone calls without being tied to a specific location. Mobile phones, as part of the cellular systems, have allowed people to break the tie between location and access to communication. Mobile phones have also allowed people to reach another who is in move. With the development in cellular systems, people are allowed to reach another who is mobile in anywhere at anytime.
The first generation of mobile communication systems were born in the early 1980s. The marriage of radio and telephone technologies gave birth to mobile phones and triggered a turning point in telecommunications. Adding radio access to a telephone network meant that for the first time in history, the concept of a telephone being at a fixed point in the network was no longer valid. The benefits of being able to make and receive telephone calls anywhere had appeal to business peoplexe2x80x94the original market. In the first generation of cellular networks, analog wireless technology were used for the user connection (called the xe2x80x9cair interfacexe2x80x9d). Every voice channel had its own narrow frequency band, using a technology called Frequency Division Multiple Access (FDMA).
However, as the demand for mobile phones grew and grew, regularly exceeding forecasts, it became obvious that the available radio spectrum would not be adequate to accommodate the expected numbers of mobile phone users. The digital technology became the solution to the problem. The answer lay in new digital wireless technologies that allow larger numbers of mobile subscribers to be supported within a given frequency allocation. Time Division Multiple Access (TDMA) technology is used in which a broader frequency channel is divided into intermittent time-slots, i.e. several calls share the same frequency channel at any one time. The digital technology also offered other important benefits. It provided better voice quality and improved security against unauthorized eavesdropping. Another technology, Code Division Multiple Access (CDMA) has also been developed subsequently to increase capacity.
The first and second generation mobile communication systems were mainly set to support voice communications, although today""s mobile phones can also be used for data transfer at rates that are acceptable for relatively low-speed data applications such as sending and receiving of faxes and e-mail. However, these systems do not support high-speed data or video applications. The third generation mobile communication system is being developed to remove the bandwidth bottleneck and support a whole new range of voice, data, video, and multimedia services. For example, smart messaging is bringing Internet services to every mobile user""s fingertips. As people become used to the freedom that mobile communications have provided, they will become more demanding about the information and services required to benefit their lives.
The demand by consumers all over the world for mobile communications service continues to expand at a rapid pace and will continue to do so for at least the next decade. To satisfy such demand, more and more innovative mobile telecommunications networks are being built in this growing industry.
Code Division Multiple Access (CDMA) is emerging as one of the main technologies for the implementation of third-generation (3G) cellular systems. In CDMA, each user is assigned a unique code sequence (spreading code) that is used to encode an information bearing signal. The receiver, knowing the code sequences of the user, decodes a received signal after reception and recovers the original data. This is possible since the cross-correlations between the code of the desired user and the codes of the other users are small. Since the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information-bearing signal, the encoding process spreads the spectrum of the signal and is therefore also know as spread-spectrum modulation.
CDMA may be classified according to the modulation techniques. For example, the system may a direct sequence (DS) spread-spectrum CDMA system wherein the information bearing signal is multiplied directly by a high chip rate spreading code. Another modulation technique is frequency hopping spread-spectrum wherein the carrier frequency at which the information-bearing signal is transmitted is rapidly changed according to the spreading code. Time hopping spread-spectrum involves transmitting the information-bearing signal in short bursts rather than continuously wherein the timing of the short bursts are decided by the spreading code. Hybrid modulation is also possible where two or more of the above-mentioned modulation techniques are used. Moreover, it is possible to combine CDMA with other multiple access methods: TDMA, multicarrier (MC) or multitone (MT) modulation.
In DS-CDMA, the modulated information-bearing signal (the data signal) is directly modulated by a digital, discrete time, discrete valued code signal. The data signal may be either an analog signal or a digital one. Typically it is a digital signal. For a digital signal, the data modulation is often omitted and the data signal is directly multiplied by the code signal and the resulting signal modulates a wideband carrier. CDMA system often use a hybrid diversity scheme to capture both strong and weak signals in the same cellular region. To capture both the strong and weak signals in the same cellular region antenna-array diversity and RAKE diversity are implemented. Further, RAKE receivers for both the mobile and base stations are specified to improve reception in the cases where the delay spreads are significant.
Current CDMA receivers are based on the RAKE receiver principle, which considers other users"" signals as interference. However, in an optimum receiver al signals would be detected jointly or interference from other signals would be removed by subtracting them from the desired signal. This is possible because the correlation properties between signals are known (i.e., the interference is deterministic not random).
The capacity of a direct sequence CDMA system using RAKE receiver is interference limited. In practice this means that when a new user, or interferer, enters the network, other users service quality will degrade. The more the network can resist interference the more users can be served. Multiple access interference that disturbs a base or mobile station is a sum of both intra- and intercell interference.
Multiuser detection (MUD), also called joint detection and interference cancellation (IC), provides means of reducing the effect of multiple access interference, and hence increase the system capacity. In the first place, MUD is considered to cancel only the intra-cell interference, meaning that in a practical system the capacity will be limited by the efficiently of the algorithm and the intercell interference.
In addition to capacity improvement, MUD alleviates the near/far problem typical to DS-CDMA systems. A mobile station close to a base station may block the whole cell traffic by using too high a transmission power. If this user is detected first and subtracted from the input signal, the other users do not see the interference.
The conventional matched filter bank method in a multiuser detector experiences MAI (Multiple Access Interference) and the near-far problem. Optimal multiuser detector that have been proposed can eliminate the MAI and offer a significant improvement over the conventional multiuser detector. However, for a K-user, N-bit communication system, it requires 2NK times exhaustive searches to find a maximum likelihood sequence, which is computational intensive. This has lead researchers to use sub-optimum multiuser detectors, such as decorrelating detectors and minimum mean-squared error (MMSE) detectors, which require the calculation of the inverse of the cross-correlation matrix or the matrix which has the same scale.
The other group of multiuser detectors is based upon interference cancellation (IC). The idea is to subtract the interference generated by users other than the desired user. Lower computation demanding and hardware related structures are the major advantages of this strategy. One of the most effective IC is the parallel interference cancellation (PIC) which comes from the iterative multistage method, which was first proposed by M. K. Varanasi and B. Aazhang, in xe2x80x9cMultistage Detection in Asynchronous Code Division Multiple Access Communicationsxe2x80x9d, IEEE Transactions in Communications, Vol. 33, NO. 4: 509-519, Apr. 1990.
The inputs of one particular stage are the estimated solution of previous stage. After interference cancellation, the new estimations, which should be closer to the transmitted bits, come out to be fed into the next stage. Almost all existing multistage based algorithms neglect the fact that as the iterations progress, the solution becomes more and more invariant, i.e. more and more elements in the output vector turn out to be the same as the elements in the input vector. Ideally at the last iteration stage, the output and the input should be identical if the algorithm converges. Therefore in last several stages, the multistage detector will almost calculate from the same input to get the same result. This is a substantial waste of the computation power and it increases the system delay.
It can be seen that there is a need for a multistage detector that maximizes computation power while minimizing system delay.
It can also be seen that there is a need for a method and apparatus for providing differencing multistage detection in the reverse link of a Code Division Multiple Access communication system.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an improved multistage detector in wireless communication systems.
The present invention solves the above-described problems by providing a multistage detector that maximizes computation power while minimizing system delay. The differencing multistage detector achieves both high performance in the interference cancellation and computational efficiency, which leads to a very large scale integrated circuit (VLSI) implementation. When the iterative algorithm of the differencing multistage detector converges, the difference of the solution vectors between two consecutive stages is mostly zero.
A system in accordance with the principles of the present invention includes a differencing multistage detector for receiving signals from a plurality of users in a cell of a communications system, the differencing multistage detector reducing the effect of multiple access interference to a signal from a desired user caused by interference from other users in the cell, wherein the differencing multistage detector includes a plurality of stages, each stage including an interference canceller for removing intra-cell interference caused by the other users in the cell and producing an estimation output vector, wherein except for a first stage, the estimation output vector of a current stage is based on both a decision of the interference canceller of the current stage and the output from an interference canceller of a previous stage.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the estimation output vector of a current stage is produced by subtracting the output from an interference canceller of a previous stage from the decision of the interference canceller of the current stage.
Another aspect of the present invention is that except for the first stage each interference canceller calculates an estimate of multi-user interference by computing a product of a cross-correlation of the received signals and the difference signal.
Another aspect of the present invention is that the difference signal comprises 0, +2, or xe2x88x922.
Another aspect of the present invention is that the computing of the product is omitted when the difference signal is 0, and the computing of the product is performed by storing the cross-correlation of the received signals in a register and shifting the bits one place forward when the difference signal is +2 and one place forward with a sign change when the difference is xe2x88x922.
Another aspect of the present invention is that the interference canceller is a parallel interference canceller.
Another aspect of the present invention is that control between stages is handled by a handshaking protocol.
Another aspect of the present invention is that the input to each stage is in two""s compliment form.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.